This invention relates to an electrochromic device. In particular, it relates to a nanostructured metal oxide film, doped to metallic levels of conductivity and modified by chemisorption of an electroactive compound, an electrode comprising such a nanostructured film and an electrochromic device comprising such an electrode.
The use of electrochromic devices in applications where optical modulation is required, such as large area static displays and automatically dimmable rear-view mirrors, is well known. Electrochromic devices comprising at least one electrode incorporating a semiconducting nanostructured metal oxide film modified by chemisorption of an electroactive compound are also known, see for example WO-A-97/35227 and WO-A-98/35267.
The device disclosed in WO-A-97/35227 comprises an n- or p-type redox chromophore chemisorbed at the surface of a nanostructured semiconductor electrode, and an auxiliary electroactive compound of the p- or n-type, respectively, which can be oxidised or reduced in a reversible manner, is dissolved in the electrolyte. In the device disclosed in WO-A-98/35267 an n-type redox chrompohore is chemisorbed at the surface of a nanostructured semiconductor electrode and a p-type redox promoter is dissolved in the electrolyte. Although the switching time of these devices is more rapid than that of previously known devices, it is still relatively slow due to the rate-limiting step being the diffusion of the electroactive compound in electrolyte to the relevant electrode. Attempts to eliminate this rate-determining step by adsorbing this compound to the electrode to which it diffuses have only resulted in moderate increases in the rate of switching due to the semiconducting nature of the nanostructured substrate.
While the devices disclosed in WO-A-97/35227 and WO-A-98/35267 are adequate for the applications mentioned above, more rapid switching times would be desirable, especially where dynamic displays, privacy glazing and smart windows are concerned.